It is known that metal-oxide-semiconductor field-effect transistors (MOSFETs) have certain electrical characteristics which vary with time and voltage bias. These variations in electrical characteristics are particularly noticeable in large gate area MOSFETs, such as power MOSFETs. For example, one such electrical characteristic, the turn-on voltage or threshold voltage of an MOSFET, has been measured to vary over 500 millivolts from an exemplary desired voltage of two volts. This amount of variation can make circuits using these devices inoperable. The most widely recognized reason for the variations results from mobile ion contamination of the MOSFET by mobile ions present in dielectrics. Typically the source of such contamination is ionized sodium (Na.sup.+) found in silicon dioxide, which is used as gate insulator material for the MOSFETs and as the insulator between adjacent conductors in integrated circuits.
The mobile ions in the silicon dioxide gate insulator shift the threshold voltage of the corresponding MOSFET due to the charge of the mobile ions. This effect can be understood with reference to FIG. 3 which shows a portion of the exemplary MOSFET 1. Mobile ions (Na.sup.+) migrate from several sources (one of which is the conductor metal 6, discussed below) through the gate insulator 3 (e.g., silicon dioxide) toward the gate 4. Some of the ions remain between the gate 4 and the MOSFET 1 channel (not shown) in the p layer. The charge on the mobile ions accumulates under the gate 4, shifting the gate voltage required to turn on the MOSFET 1 (threshold voltage) negatively, i.e., the MOSFET 1 acts as if a positive bias voltage were permanently applied to the gate 4 thereof. The number of mobile ions Na.sup.+ under the gate 4 varies with the magnitude of the bias voltage applied to the MOSFET 1 and the length of time the bias voltage is applied. A layer of phosphorus-doped glass (P-glass) 5 is deposited over the gate 4 to trap (getter) mobile ions that come in contact with it from overlying layers (not shown), the source of mobile ions resulting mainly from contaminated manufacturing equipment. However, a second source of the mobile ions is the aluminum conductor 6, shown here contacting the n+ source (or drain) of the MOSFET 1. Since the conductor 6 is in direct contact with the insulator 3, the P-glass 5 cannot getter all the mobile ions added to the insulator 3 from the conductor 6.
In the manufacture of MOSFET 1, the source of aluminum for the conductor 6 is typically an aluminum target, having added thereto a small amount of copper to reduce electromigration of the aluminum at high current densities. Other metals may also be alloyed with the aluminum, such as silicon and titanium. The aluminum target is used in a sputtering apparatus in which the aluminum and copper target is slowly vaporized and deposited onto a workpiece, such as a wafer having integrated circuits or discrete devices thereon. This process is typically referred to as sputtering. If the target or the chamber used for sputtering is contaminated, then mobile ions will be introduced into the MOSFET 1. One prior art approach is to use as pure aluminum-copper alloy target as possible to minimize the introduction of mobile ions into the workpiece. However, the contamination in the sputtering chamber remains and it is difficult to obtain very high purity targets at reasonable cost. Another technique involves modifying the fabrication process such that the conductor 6 never comes into contact with the oxide 3 and the P-glass 5 completely surrounds the conductor 6. This technique complicates the fabrication process, reducing the yield of operational devices and increasing the manufacturing costs thereof.
Another deleterious effect of mobile ion contamination on integrated circuits is the formation of low conductivity paths through the silicon dioxide insulator between adjacent conductors in an integrated circuit. This effect is particularly troublesome where two conductors in different levels of metallization cross.